Gasoline obtained by catalytic cracking requires further processing before it can satisfactorily meet the present day stringent requirements for high octane and low sulfur content. Thus, catalytically cracked gasoline has a comparatively high olefin content, a low aromatics content and if there has been no initial treatment of the feedstock, an unacceptable high sulfur content. Quality improvement may be carried out by catalytic reforming with platinum-containing reforming catalysts. However, the presence of sulfur- and nitrogen-containing compounds in the reformer feedstock reduces the catalyst performance and removal of such compounds by catalytic hydrotreatment is considered necessary prior to reforming, with a consequent increase in cost.
It has been proposed in accordance with European Patent No. A-271264 to employ a platinum-containing Y-type zeolite catalyst in a single stage process for reducing the sulphur content and increasing the octane number of an olefin-containing feedstock. However, as mentioned above, platinum-containing catalysts are readily poisoned by sulfur- and nitrogen-containing compounds. They are also expensive and difficult to regenerate. There is, therefore, a need to provide an alternative upgrading process which does not rely on platinum and which can be used with sulfur-containing feedstocks without the requirement for initial hydrotreatment.
European Patent No. A-252705 desribes a process for producing aromatic compounds from feedstocks containing at least 50% wt C.sub.2-12 aliphatic hydrocarbons, using a gallium-containing ZSM-5 type catalyst having a silica/alumina mole ratio greater than 10. The process is exemplified solely by the aromatization of n-hexane to an aromatics-containing product, the aromatics selectivity being increased when the catalyst has been prepared by insertion of the gallium into the framework of the zeolite under alkaline conditions. European Patent No. A-252705 contains no teaching to suggest that the process would be applicable to a sulfur-containing commercial mixed feedstock such as gasoline obtained by catalytic cracking, let alone teaching to the effect that use of such a catalyst would promote the simultaneous aromatization and desulfurization of a sulfur-containing feedstock. Indeed the teaching of the above-mentioned European Patent No. A-271264, which shares the same applicant as EP-A-252705, is that, although many crystalline silicate zeolites are now known to the prior art, it is necessary to resort to a noble metal-containing crystalline zeolite catalyst in order to effect simultaneous reduction of sulfur content and increase of octane when processing an olefinic gasoline from cracking processes.
It has now been found that a (mixed) feedstock containing an unacceptable high portion of sulfur and substantially boiling in the gasoline range, suitably a feedstock derived from catalytic cracking, can very attractively be upgraded in respect of both sulfur-content and octane quality in a two-stage process which does not rely on noble metals and whereby, in both stages, use is made of a catalyst comprising a crystalline (metallo)silicate having a specific X-ray diffraction pattern.